Remote sensing of forest succession in Hong Kong's country parks

Abstract

This study aimed to use remotely sensed images to investigate the dynamics of secondary forest succession after a complete clearance of forest in Hong Kong during WWII. The study area (~2800 ha), included Tai Mo Shan and Shing Mun Country Parks in the NewTerritories of Hong Kong. A multi-scale object-based approach was applied to sequential aerial photographs and recent high-resolution satellite images to map structural changes in natural vegetation over the last 70 years, from 1945 to 2014. Temporal changes in the spatial extents of structural classes, and rates of change were determined for each of the periods; from 1945 to 1963, 1963 to 1989, 1989 to 2001, and 2001 to 2014. Also, spatial patterns of forest succession related to topography and morphology of the landscape were analyzed to determine the influence of changes in specific landscape structural parameters on forest succession. The mapping of structural changes associated with forest recovery over the past 70 years also enabled the recording and study of changes in species composition along the successional and environmental gradients for better understanding of the processes of ecosystem recovery in the degraded tropical landscape. Therefore, a forest inventory data consisting of 28 quadrats (20 x 20 m) was collected to analyze patterns of species composition associated with different stages of the succession, as well as explain the influence of topography, soil properties, and age of forest on the species assemblages. Spatial analysis of forest succession showed that natural forest has established rapidly in areas where a shrub cover was able to colonize in fire protected grasslands. The fastest period of forest regeneration, at 10.9% annually between 1989 and 2001, occurred when the landscape achieved greatest habitat diversity and juxtapositioning of habitat patches. This rapid regeneration occurred by infilling from the remnant forest in adjacent valleys rather than by an advance along a broad forest front, and led to a simplification of the landscape and declining habitat diversity. After 2001, succession to forest was slower, at 7.8% annually, as forest patches consolidated and edge habitats reduced. Significantly, the birds of open habitats including bulbuls and hwamei, known to disperse seed in the study area, may become less effective as forest patches consolidate, and this is a concern, given the loss of most forest mammals. Consequently, progression to a mature, biodiverse and stable forest ecosystem may depend on dispersal agents other than those which have operated over recent decades. Additionally, the practice of afforestation as a nursery stage on degraded hillsides, for the establishment of forest seedlings by natural invasion, is not supported by the evidence, as when the native Pinus massoniana plantations were eliminated by disease during the 1970s, no forest or woody species were seen in the areas affected. In fact, there was a reversion to grassland, which persisted there for almost three decades, until recent shrub invasion. Secondary forest successions in Hong Kong starts with the transition of fire protected grassland into shrubland and colonization of early successional tree species that pave the way for intermediate or late successional tree species. Analysis of community data along the environmental gradients indicated that the topography and successional stages play an important role in the distributional patterns of species assemblages in the study area. At the landscape scale, elevation divided the species composition into two components, and these were further distinguished by successional stage at localized scale. Micro scale variations were further explained by convexity of terrain. Forest at higher elevation was found to be more diverse and had markedly higher concentration of soil carbon relative to nitrogen. Therefore, for restoration of the degraded ecosystem, it is recommended to recognize the distinctiveness of ecosystems at these two different elevations in the study area. Additionally, effective forest management policies could include sowing of native shrubs extending linearly from established forest, to maximize edge length between woody species and grasslands for better colonization, and planting of late successional species in areas where forest pioneers are dying out. Thus, for effective forest restoration, the roles of GIS and Remote Sensing are important in locating sites for assisting shrub encroachment, as well as for accelerating secondary succession where shrubland has already been established, and for managing natural succession by planting late successional tree species where the oldest forest pioneers are established.